The present invention relates to novel ion exchange agents and the manufacture and use thereof. In another aspect, the present invention relates to a novel method of contacting solid ion exchange agents with liquids. Yet another aspect of the present invention relates to the recovery of mineral values from mineral-containing deposits. A still further aspect of the present invention relates to an improved method of leaching and recovering mineral values from mineral-containing ores.
Ion exchange materials, their manufacture and their use are well known in the prior art.
It is also well known that there are two types of ion exchange materials, which depend upon their physical form, namely, solid or liquid. The solid ion exchange materials are semi-rigid gels prepared as spherical beads. These materials are generally referred to in the art as ion exchange resins and the use thereof to remove ions from the solutions is often referred to as adsorption. By contrast, liquid ion exchange materials are often referred to in the art as solvents and the use thereof to remove ions from a solution is referred to as solvent extraction. Obviously, references to liquid ion exchange materials as solvents and their uses as solvent extraction is misleading to the extent that this terminology is too broad and it is not technically accurate. Consequently, the terminology "solvents" and "solvent extraction" will be avoided in the present specification and both solid and liquid ion exchange materials will be referred to herein as "ion exchange agents" or "ionic agents" and the use of both solid and liquid ion exchange agents will be referred to as "ion exchange".
There are also two types of ion exchange agents, which depend upon the chemical characteristics thereof. Either solid or liquid ion exchange agents may exist in the form of anionic agents or cationic agents. Accordingly, ion exchange agents are selected on the basis of the type of ion to be extracted from the solution to be treated.
The preparation of liquid ionic exchange agents is of course a straightforward formation of the appropriate compounds. On the other hand, the preparation of solid ion exchange agents requires specific techniques in order to produce a material suitable for use as an ion exchange agent. Generally, solid ion exchange agents are, for example, polymers of monomeric material, such as styrene, and copolymers of materials, such as styrene and divinylbenzene. In either case, the liquid monomers are charged to a reactor containing water as a continuous phase. The monomers are water-insoluble so that agitation suspends the monomers as droplets in the water phase. The reactor is heated to a temperature sufficient to cause the monomers to polymerize or copolymerize, as the case may be, thus forming solid thermoset plastic spheres. This technique for the formation of solid ionic exchange agents is generally referred to in the art as "suspension polymerization". While water is specifically referred to as the continuous phase above, other suspending media which are immiscible with the monomer or in which the monomer is essentially insoluble may also be utilized. This basic technique of suspension polymerization results in the formation of gellular particles or beads wherein the pore structure is defined by molecular-sized openings between polymer chains. This type of molecular porosity is generally referred to in the art as "microporosity or microreticularity". There are also known techniques of "macroreticular" resins, which by contrast, contain significant non-gel porosity in addition to the normal gel porosity. The non-gel pores are channels between agglomerates of minute spherical gel particles. The microreticular resins make a continuous polymeric phase, while the macroreticular resins or agglomerates are randomly packed microspheres with a continuous non-gel porous structure. The macroreticular resins may be prepared for example, by adding to the suspension previously described a suitable "polymer precipitant". Such precipitants are solvents for the monomer or monomer mixture being polymerized, but exert essentially no solvent action on the resultant polymer or copolymer nor are they imbibed in the copolymer or polymer to any appreciable extent. These precipitants are in general, alkanols having about 4 to 10 of carbon atoms per molecule, such as tertiary amyl alcohol, secondary butanol, etc. Details of the conventional techniques for the formation of microreticular ion exchange agents and, more specifically, the formation of macroreticular ion exchange agents are set forth in, for example, U.S. Pat. No. 4,224,415, which is incorporated herein by reference.
For ion exchange purposes in general, it is important that a major portion of the particles or beads, for example 80% or more by weight, be in the form of granules of 10 to 60 and preferably 20 to 40 mesh size. (Here and in the discussion below, mesh sizes referred to are Tyler Standard Screen Scale Sieve mesh sizes, well-known to those skilled in the art of ore leaching and ion exchange agent manufacture and use.) Smaller particles tend to be washed away during conditioning and use, whereas larger particles tend to develop excessive internal strains and to undergo shattering and spalling with the formation of fine particles, both during preconditioning and use.
Conversion of the solid plastic particles or beads to an ion exchange material is also well known in the art. For example, cation exchange materials may be produced by sulfonation of the beads with concentrated sulfuric acid, oleum, sulfur trioxide or chlorosulfonic acid. Anion exchange agents may be prepared by chloralkylating and subsequently aminating the polymers. For example, the polymer may be treated with chloromethylether and thereafter with either dimethylamine (producing a weak base ion exchange agent) or trimethylamine (producing a strong base ion exchange agent).
There are also a number of known uses for both liquid and solid ion exchange agents, such as the removal of ions from sea waters, drinking waters, etc., run off water from mines and mine tailings and the extraction of minerals from leachants or lixiviants utilized for the extraction of metals from subsurface formations and ores. Certain specific embodiments of the present invention are direction to the removal of minerals from subsurface formations and ores and accordingly a more detailed description of the same will be set forth hereinafter.
Removal of ions from an ion-containing solution, whether with liquid or solid ion exchange agents is basically a matter of contacting the solution with the ion exchange agent and thereafter separating the ion exchange agent from the deionized solution. In the case of liquid ion exchange resins, contacting may be batch, semi-continuous, continuous or continuous countercurrent. In addition to the ability to carry out a countercurrent contacting, liquid ion exchange agents have a number of advantages, including high efficiency, low capital cost, adaptability to automatic control and the availability of nearly "off the shelf" equipment. However, disadvantages include the necessity of nearly solids-free ion-containing solutions, the formation of emulsions, the loss of solvent to the deionized solution and, in some cases, extra safety precautions in the handling of flammable fluids. In the use of solid ion exchange agents, contacting is carried out either in a batch or semi-continuous type of process, while continuous countercurrent operation is most difficult. In any event, plants for the use of solid ion exchange materials are relatively complex and require specialized equipment. While solid ion exchange materials can tolerate solids in the ion containing material, in such cases the particle size of the ion exchange material must be carefully controlled for the reasons previously set forth and there is a tendency to plug the ion exchange material.
Since ion exchange agents are organic in nature, separation of the ion exchange agent from the deionized solution can be carried out by gravity separation in the case of either liquid or solid ion exchange agents and of filtering in the case of solid ion exchange agents. In any event, loss of ion exchange agent and adequate separation are serious problems.
Removal of the exchanged ions from the ion exchange agent is generically a matter of reversing the ion exchange. The removal of exchanged ions from a liquid ion exchange agent is generally referred to in the art as "stripping" and the agent utilized for this purpose is generally referred to as a "stripping agent". Stripping agents in general include nitrates, chlorides, sulfates, carbonates, hydroxides and dilute acids. In the case of solid ion exchange agents, removal of the exchanged ions is commonly referred to in the art as "elution" and the agent utilized is referred to as the "eluant". At times the term "regeneration" is also used. However, the latter terminology will be utilized herein only for the treatment of the ion exchange agent to remove "poisons" or other extraneous ions not removed in the normal process of elution so as to restore the ion exchange agent to near its original capacity. Typical elution agents include dilute nitrate or chloride solutions, as well as sulfuric acid solutions. If solids are contained in the ion-containing solution, it is usually necessary to subject the ion exchange agent to a preliminary water wash to remove the solid materials prior to elution. In both cases, there are the obvious problems of loss of ion exchange agent when any solution is passed through the ion exchange agent and particularly where other solids are also present.
As previously indicated, certain embodiments of the present invention are directed to the recovery of minerals from subsurface earth formations and ores by means of leachants or lixiviants and the solid ion exchange agents of the present invention are particularly useful in such operations.
Numerous minerals are present in subsurface earth formations in very small quantities which make their recovery extremely difficult. However, in most instances, these minerals are also extremely valuable, thereby justifying efforts to recover the same. An example of one such mineral is uranium. However, numerous other valuable minerals, such as copper, nickel, molybdenum, rhenium, silver, selenium, vanadium, thorium, gold, rare earth metals, etc., are also present in small quantities in some subsurface formations, alone and quite often associated with uranium. Consequently, the recovery of such minerals is fraught with essentially the same problems as the recovery of uranium and, in general, the same techniques for recovering uranium can also be utilized to recover such other mineral values, whether associated with uranium or occurring alone. Therefore, a discussion of the recovery of uranium will be appropriate for all such minerals.
Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vanadium, gold and the like, can be simultaneously recovered.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a "pregnant" solution, is produced through at least one production well penetrating the deposit. In certain embodiments, the present invention is directed to both in-situ leaching and the leaching of mined ores.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
Aqueous acidic solutions are normally quite effective in the extraction of uranium values. However, aqueous acidic solutions generally cannot be utilized to extract uranium values from ore or in situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. Aqueous alkaline leach solutions are applicable to many types of uranium-containing materials and for some ores are less expensive than acids.
As described to this point, the extraction of uranium values is dependent to some extent upon the economics of mining versus in-situ extraction and the relative costs of acidic leach solutions versus alkaline leach solutions. However, this is an oversimplification, to the extent that only uranium in its hexavalent state can be extracted in either acidic or alkaline leach solutions. While some uranium in its hexavalent state is present in ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such as UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.2UO.sub.3), the most prevalent variety of which is pitchblende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub.2 (UO.sub. 2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from ores with aqueous acidic or aqueous alkaline leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state.
Combinations of acids and oxidants which have been suggested by the prior art include nitric acid, hydrochloric acid or sulfuric acid, particularly sulfuric acid, in combination with air, oxygen, sodium chlorate, potassium permanganate, hydrogen peroxide and magnesium perchlorate and dioxide, as oxidants. Alkaline leachants and oxidants heretofore suggested include carbonates and/or bicarbonates of ammonium, sodium or potassium in combination with air, oxygen or hydrogen peroxide, as lixivants.
Numerous problems obviously arise in the leaching of uranium values from uranium-containing ores. One of the most obvious is, of course, the large quantities of ores being handled and treated compared with the amount of uranium recovered. Such large quantities of ores make it costly to crush and grind the same to a size which can be effectively leached in a relatively short period of time. For example, as previously pointed out, leached ore should be reduced in size to less than about 14 mesh, but an even smaller size, in the neighborhood of 100 to 400 mesh, or smaller, would be ideal. The cost of the latter, however, becomes prohibitive. It is, therefore, desirable to reduce the degree of grinding necessary. In addition, it would be highly desirable to reduce the quantities of ores handled in any given step of the process.
The large quantities of ores being treated also increase the amounts of leachant or lixivants and oxidants required in order to recover a given amount of uranium and/or attain such recovery in a reasonable time. Thus, it is also highly desirable to reduce the amounts of leachant or lixivant and oxidant to a minimum for effective results.
While the leaching operation can be carried out at temperatures from atmospheric temperature up to about the boiling point of water, it is known that for most ores the higher the temperature, the more effective and more rapid the leaching. Consequently, the usual range of temperatures is between about 80.degree. and about 100.degree. C. While this temperature range appears modest for most chemical operations, in the leaching of uranium-containing ore, the temperature becomes a very significant problem. This is true since, at the high temperatures employed, the cost of materials of construction of the leaching tanks is a major factor. For example, it is necessary to use rubber-lined stainless steel tanks and the manufacturers of such tanks will not assure reasonable lifetimes for the linings. Consequently, the utilization of less expensive equipment is desirable and even a small reduction in the temperature of the leaching operation can substantially reduce equipment costs and lengthen equipment life.
While it is relatively easy to recover 50% to 60% of the uranium content of an ore, at relatively low temperatures, with relatively low concentrations of leach solution and in relatively short periods of time, such recoveries are not acceptable except for certain very low cost in-situ operations. For an economic operation, especially when all the cost and effort have been expended to mine an ore, recoveries in excess of about 85% of the original uranium are required and usually above 90%. This, again, contributes substantially to the cost of leach solutions. Also, as in any other operation of this type, it is relatively easy to approach the desired and economic recoveries, but is most difficult to attain recovery of those last small increments which are necessary or desirable for an effective and economic oepation. Such limitations in the leaching operation are often caused by the fact that the leaching is a reversible reaction which results in an equilibrium relationship resulting between the uranium in the ore, the chemicals used to extract the uranium and the extracted uranium. Hence, regardless of the manner of contacting, etc., there is a maximum amount of uranium which can be leached without increasing the concentration of leach chemicals. In some ores there appears to be phenomena at work like organic-matter encapsulation of the mineral values. In these ores the rate of leaching is limited by diffusion of the leach chemicals into the mineral and of products of leaching back out into the bulk leach solution. Here, again, there is greater extraction in a given period of time with greater leach chemical concentration. Additionally, in both types of situations, leaching would be enhanced by reduced somehow the concentration of leaching products in the bulk solution. Such reduction is probably an important reason for the success of the methods in accordance with the present invention.
Mined uranium ores are generally crushed and then ground, for example, by the use of ball mills or rod mills. Conventionally, the ore is ground to a maximum particle size less than about 14 mesh and in some cases, less than about 300 mesh, depending upon the nature of the ore. In some cases, the ore is separated into a coarse fraction and a fines fraction. For example by screening to produce a fines fraction, of which a majority will pass through a 200 mesh screen or smaller. In some cases screening can be done to produce a fines fraction, which passes a screen in the neighborhood of 400 mesh while in other instances, the majority of the fines pass a screen in the neighborhood of about 300 mesh. However, a preferred technique for separation involves wet classification which is known to those skilled in the art of minerals recovery. In wet classification, the solids which float in the solution are separated and constituteed in the art of minerals recovery. In wet classification, the solids whihc float in the solution are separated and constituteed in the art of minerals recovery. In wet classification, the solids whihc float in the solution are separated and constitute the fines fraction, often referred to in the art as "slime" and the larger solids which settle out are referred to as the coarse fraction or "sand". By way of example, in wet classification of a particular ore, the resultant fines fraction comprised about 91% passing a 400 mesh screen in one case and about 87% passing a 325 mesh screen in another case. Such separation is usually carried out for the reason that the coarse fraction contains a smaller percentage of the original uranium than the fines fraction and it is substantially easier to leach than the fines fraction, thus different techniques for leaching can be utilized and certain economies and simplifications carried out. However, there are usually difficulties in leaching the fines fraction in spite of the fact that in the fines, the mineral values are probably better liberated from the gangue. The extra difficulty in leaching the fines is often attributable to increased leach chemical consumption by sorption onto and reaction with the fine gangue minerals.
While there are numerous techniques for recovering leached uranium or other minerals from the leach solution or lixiviant solution, one of the most widely utilized techniques is by the use of either liquid or solid ion exchange agents. The previously discussed techniques for ion exchange separation of the ion exchange agent from the deionized solution and the recovery of the exchanged ions from the ion exchange material are the same for minerals recovery as those previously discussed with relation to the utilization of ion exchange agents generally. Obviously, the same problems exist with respect to the presence of solids in a solution to be deionized, as was previously discussed, since these problems arise when either unclassified ore containing such fines, or a fines fraction alone are treated. This is particularly true since the fines or slime fraction is most difficult to remove from the leach solution. In order to overcome this problem and leach and recover uranium from a fines or slime fraction, a technique has been utilized in which the ore is first leached with a conventional leachant or lixiviant and is thereafter separated to remove the coarse fraction or sand from the fines or slime. The separated solution of fines in the leach solution is referred to as "pulp". The pulp is then contacted with a solid ion exchange agent to adsorb the uranium from the leach solution. This is a batch-type process in which a plurality of tanks are connected in series. Each tank is equipped with a plurality of baskets filled with the ion exchange agent. The baskets rise and fall in the pulp allowing it to seep through and contact the ion exchange agent which adsorbs the uranium anions. The vigorous movement of the baskets disperses the ion exchange agent in the basket and prevents the same from being caoted with slimes while the pulp freely circulates through the baskets surrendering its uranium content to the ion exchange agent. After passing through the various tanks, the barren pulp is water washed and set to a tailings pond. When the ion exchange agent becomes loaded with uranium, the tank in question is switched to an elution cycle and an eluant, such as sulfuric acid and sodium chloride is passed through the ion exchange agent to displace the uranum ions from the ion exchange agent. The uranium is then precipitated from the pregnant eluant solution. Processes of this sort are referred to as "resin-in-pulp leaching" by those skilled in the art.
Such an operation, while having some advantages also has a number of disadvantages, among which are the fact that it is a batch-type process, the ion exchange agent must be of controlled size and uniformity, attrition of the ion exchange agent is promoted and, thus, loss of the ion exchange agent and serious difficulties are encountered in maintaining separation of the ion exchange agent and the slime or fines.